Optically read and written memory devices have the potential of providing a significant advance in the data storage capacity of computer systems and the like. In principle, data storage capacities of the order of 10.sup.12 bit/m.sup.2 of storage media are possible with such devices. However, in spite of intense research and development work, optical read/write memories based on existing technologies are at best a marginal improvement over conventional magnetic memories.
The most attractive of the currently available read/write optical disks are based on magneto-optic materials. These disks have latency times of about 100 ms and may be rewritten only about 10.sup.4 times. For comparison, conventional magnetic hard disks used in computer systems may be rewritten more than 10.sup.6 times and have latency times less than 20 ms. In addition to poor latency times and insufficient rewrite capability, the intensity of laser light required to store the information in magneto-optical media is of the order of 10.sup.5 watts/cm.sup.2. This high intensity results from the need to thermally modulate the magneto-optical media. To operate at these power densities, the laser diode used to write the magneto-optical media must operate near saturation. As a result, the lift-time of the laser diode is significantly reduced.
Most prior art magneto-optical media based memories have been constructed in the form of read/write optical disks. These disks are read by observing alterations in a laser beam which passes through the magneto-optical media. The preferred means of detecting the data is to observe the rotation of the polarization of the laser light after the light has passed through the media once, bounced off of a reflective coating on one side of the magneto-optical media, and then passed back through the media for a second time. As a result, the read/write head of the optical disk must include a laser diode for generating the light beam, a polarization analyzer, and a light detector. The weight of these components limits the seek time of the disk arm which further reduces the performance of such disk drives.
Another problem with prior art magneto-optical memories is the use of phase transitions for the storage of the information. In such memories, a bit of information is stored in a small area of the disk. One binary state corresponds to the media in this area being in a first phase while the other binary state corresponds to the media being in a second phase. The transition between states is induced either magnetically or optically. In either case, the magneto-optical media must be operated near a phase boundary. However, the location of the phase boundary is, in general, sensitive to environmental variables such as temperature, physical stress, and humidity. As a result, the optical memories are only operational in a narrow range of environmental variables. Such limitations have hampered the development of practical read/write optical disks.
Accordingly, it is an object of the present invention to provide an improved read/write optical disk.
It is a further object of the present invention to provide a read/write optical disk with lower latency times than currently available read/write optical disks.
It is yet another object of the present invention to provide a read/write optical disk which can be rewritten as many times as currently available magnetic hard disks.
It is a still further object of the present invention to provide a read/write optical disk which requires less power to write then prior art read/write optical disks.
It is a yet another object of the present invention to provide a read/write optical disk with a lighter weight reading mechanism than that of prior art read/write optical disks.
It is yet a still further object of the present invention to provide a read/write optical disk which does not require that the storage media be operated near a phase boundary.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.